The present invention relates to a phase controller of horizontal drive pulses fed to a horizontal deflection circuit in a video display device such as a television receiver (hereinafter referred to as a TV), and a method of controlling the horizontal drive pulses.
The digital broadcasting system recently drawing public attention requires video display devices including TVs to equip a function, i.e. dealing with input signals of various formats of multi-media, as one of important functions.
A fundamental function of the video display devices dealing with videoinput-signals of various formats is to display the respective video-input-signals with various horizontal frequencies at a given horizontal position in a stable manner.
A horizontal automatic-frequency-control (AFC) circuit has been used in order to display the video-input-signal at a given horizontal position. The horizontal AFC circuit compares respective phases of a horizontal sync signal and a horizontal deflection pulse (sometimes referred to as a flyback pulse)xe2x80x94the horizontal sync signal is separated from the video-input-signalxe2x80x94thereby producing a horizontal drive pulse in-sync with the horizontal sync signal. The horizontal AFC circuit thus has a phase control loop. Japanese Patent Application Non-Examined Publication No. H08-51556 discloses an example of this structure.
The video display devices dealing with video-input-signals of various formats should control correctly a phase of the horizontal drive pulse when a video-signal of any format is input.
A desirable phase difference between the horizontal drive pulse and the horizontal deflection pulse depends on a format of the video-input-signal.
The phase difference between the horizontal sync signal and the horizontal deflection pulse should be desirably adjusted responsive to video signals in respective formats by controlling the phase of the horizontal drive pulse.
The present invention aims to realize the desirable function discussed above. A phase controller, of the present invention, of a horizontal drive pulse supplied to a horizontal deflection circuit, comprises the following elements:
(a) a frequency discriminator for identifying which format of a video signal is input by detecting a frequency of a horizontal sync signal;
(b) a reference phase generator for generating a reference phase signal based on an output from the frequency discriminator;
(c) a sawtooth waveform generator for generating a sawtooth waveform signal responsive to the output from the frequency discriminator;
(d) a phase difference voltage detector for outputting a phase difference voltage responsive to a phase difference between the reference phase signal and a horizontal deflection pulse;
(e) a phase control signal generator for generating a phase control signal by comparing the phase difference voltage with the sawtooth waveform signal; and
(f) a horizontal drive pulse generating circuit for outputting a horizontal drive pulse having a phase responsive to the phase control signal.
The structure discussed above identifies which format of a video signal is input by detecting a frequency of the horizontal sync signal, thereby controlling the horizontal deflection pulse to carry the phases proper for the video-input-signal.
This phase controller of the horizontal drive pulse can further comprise the following elements:
(g) a substitutive-signal generating circuit for generating a signal substitutive for the horizontal drive pulse, and the substitutive signal being fed into the horizontal deflection circuit;
(h) a horizontal output control circuit for selecting one of the horizontal drive pulse or the substitutive signal depending on the stability of the horizontal sync signal; and
(i) a switcher for outputting one of the horizontal drive pulse or the substitutive signal by switching therebetween responsive to an output from the horizontal output control circuit.
The structure discussed above allows the horizontal deflection circuit to avoid an input of an unstable horizontal drive pulse when the horizontal drive pulse is not satisfactorily controlled due to an unstable horizontal sync signal, so that the horizontal deflection circuit avoids an unstable operation.